Figure 4.4 shows ∆Tb, the scan angle dependent asymmetry in mean bright-ness temperature, as a function of nadir viewing angle for the AMSU-B surface channels. This is the data version available from CLASS, i.e., the RFI correc-tion coefficients available in the level 1b data were applied. Different sub-plots display results for different satellites and channels. The different lines represent different three-monthly time periods of the data, the lines for recent data are darker. As a help in interpreting these curves, rough estimates of the radiomet-ric noise for each channel are displayed as horizontal lines. Noise estimates for NOAA-15 and 16 are fromBuehler et al.[2004], noise estimates for NOAA-17 are copied from NOAA-16.
To check the influence of different surface types, data were sorted into land and sea data. For the AMSU-B surface channels 16 and 17 the results depend somewhat on the surface type, therefore, both classes are shown separately. The dependence of the asymmetry curve on the surface type for the surface channels indicates that the assumption that inhomogeneities are random and, therefore, average out does not hold completely for these channels.
As expected, for the sounding channels both classes lead to similar results.
Therefore, the data was not separated into land and sea, so only global data are shown in Figure 4.5. The further discussion will, therefore, focus on the three sounding channels.
In general, observations close to nadir (viewing angle 0◦) have the smallest asymmetries, as expected. The largest asymmetries tend to occur towards the edge of the scan, but not always directly at the edge. The figure confirms that the simple method used to calculate the asymmetry is stable, in the sense that the shape of the asymmetry curve for each satellite and channel is remarkably similar for the different time periods studied, except for a slow evolution in some cases. I will come back to the time evolution of the asymmetry later.
The different asymmetry shapes for the same channels of different instru-ments suggest that each instrument has its particular signature. The oldest AMSU-B instrument on NOAA-15 is most strongly affected by scan asym-metries. Its sounding channels 18 to 20 suffer significantly, with asymmetries reaching∆Tbvalues of approximately 1.9, 9.7, and 3.0 K, respectively. Hence, particularly the newer data from Channel 19 of this instrument should be used only with caution.
The AMSU-B instrument on NOAA-16 is much less affected by scan
asym-4.3 Results and Discussion 27
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N15, CH16, Land
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N15, CH16, Sea
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N15, CH17, Land
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N15, CH17, Sea
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N16, CH16, Land
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N16, CH16, Sea
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N16, CH17, Land
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N16, CH17, Sea
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N17, CH16, Land
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N17, CH16, Sea
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N17, CH17, Land
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N17, CH17, Sea
Figure 4.4: The asymmetry ∆Tb (left side minus right side of the scan-line in flight direction) as a function of viewing angle. Nadir is at zero degree viewing angle. The rows of plots from top to bottom corre-spond to the satellites NOAA-15, 16, and 17. The first two columns from left to right correspond to the AMSU-B Channel 16 where the analysis was made for the data over the land and sea only. The next columns are the same, but for the the AMSU-B Channel 17. The dif-ferent lines correspond to difdif-ferent three-monthly time periods, the lines for recent data are darker. Horizontal lines indicate the noise equivalent temperature.
0 10 20 30 40 50 Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
n15, CH18
0 10 20 30 40 50
Viewing angle [degrees]
-10 -8 -6 -4 -2 0 2
∆Tb [K]
N15, CH19
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N15, CH20
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
n16, CH18
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N16, CH19
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N16, CH20
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N17, CH18
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N17, CH19
0 10 20 30 40 50
Viewing angle [degrees]
-2 -1 0 1 2
∆Tb [K]
N17, CH20
Figure 4.5: The same as in Figure 4.4, but for the AMSU-B channels 18, 19, and 20. Note that there is no separation of the data into the land and sea as in Figure 4.4. Also note that the y-axis scale for Channel 19 on NOAA-15 is different from the others.
4.3 Results and Discussion 29
0 10 20 30 40 50
Viewing angle [degrees]
-6 -4 -2 0 2 4
∆Tb [K]
N15, CH16
0 10 20 30 40 50
Viewing angle [degrees]
-6 -4 -2 0 2 4
∆Tb [K]
N15, CH17
0 10 20 30 40 50
Viewing angle [degrees]
-6 -4 -2 0 2 4
∆Tb [K]
N15, CH18
0 10 20 30 40 50
Viewing angle [degrees]
-6 -4 -2 0 2 4
∆Tb [K]
N15, CH19
0 10 20 30 40 50
Viewing angle [degrees]
-6 -4 -2 0 2 4
∆Tb [K]
N15, CH20
Figure 4.6: It is the same as in Figure 4.5, but shows the scan asymmetry for NOAA-15, 1999.
metries. The∆Tbvalues are mostly below 1 K, except for the edge of the scan for Channel 19, where they reach 1.4 K and for Channel 20, which shows significant asymmetries reaching negative values down to almost 1 K and positive values of approximately 2 K. As a rough help in interpreting these numbers, consider that according to Buehler and John [2005] a brightness temperature difference of 1 K for Channel 18 corresponds approximately to a relative difference in upper tropospheric relative humidity of 7%. Sensitivities can be assumed to be of the same order of magnitude for the other humidity sounding channels 19 and 20.
Hence, the asymmetry in Channels 19 and 20 is large enough to introduce sig-nificant humidity errors, if neglected. In contrast, the asymmetry in Channel 18, with an absolute value below 0.5 K, can most likely be safely ignored compared to other sources of uncertainty.
For the AMSU-B instrument on NOAA-17 only preliminary conclusions can
be drawn, since not as much data are available as for the other instruments.
The data available so far look very promising, with the asymmetry values even smaller than for NOAA-16. The only notable exception here is Channel 20, which can have asymmetries up to 2.3 K.
As it was mentioned before, the AMSU-B instrument on board NOAA-15 suf-fered strongly from the radio frequency interference (RFI) right after the launch in 1998 [Atkinson, 2001]. The scan asymmetry during this time is expected to be high. Figure 4.6 shows the scan asymmetry of the AMSU-B instrument on board NOAA-15 for spring, summer, autumn, and winter 1999 (March 1999 to February 2000). In the first half of 1999 (gray lines in Figure 4.6) there obvi-ously is an unrealistic bias in all the channels of the AMSU-B. However, starting from the autumn of 1999 (dark lines in Figure 4.6) the RFI problem seems to be corrected for at least Channel 18 – its scan asymmetry bias is within the instru-ment noise. Nevertheless, data prior to 2000 were not taken into account in this study.
While the development of the asymmetry with time has a random nature for some channels, for others the asymmetry is steadily increasing with time. For example, one can see from Figure 4.5 that the asymmetry for Channel 19 of NOAA-15 has significantly increased to -9.7 K for the most recent data. One can observe the same behavior for Channel 18 of the same instrument. To show the evolution more clearly, Figure 4.7 shows the maximum asymmetry for each three-monthly time period as a function of time for each channel and instrument.
The most striking feature of this figure is the steadily increasing asymmetry for the AMSU-B instrument on NOAA-15. It can be observed for all channels except Channel 16. The increase can be approximately described by a linear trend, as indicated in the figure. The most likely cause of this problem is a change in the characteristics of the radio frequency interference that the AMSU-B instrument on this satellite has experienced from the start, as mentioned ear-lier. The figure also shows the impact of different RFI correction coefficients.
The coefficients published byNOAA[2000] on August 13, 2004 do significantly reduce the asymmetry. Note that these coefficients were not implemented in the 1b data, so they have to be manually applied. As shown by Figure 4.7, the latest coefficients also significantly reduce the asymmetry in data before August 13, 2004. From this study, the use of the August 13, 2004 coefficients for Channel 19 data later than March 1, 2003 is recommended.
Compared to NOAA-15, the instruments on NOAA-16 and 17 show no sig-nificant trends. The apparent trends for Channels 19 and 20 on NOAA-17 are not significant because the time series of data is not yet long enough.
4.3 Results and Discussion 31
2000 2001 2002 2003 2004 2005 2006 Time [year]
-2 -1 0 1 2
max(∆Tb) [K]
CH16
NOAA 15 NOAA 16 NOAA 17
2000 2001 2002 2003 2004 2005 2006 Time [year]
-2 -1 0 1 2
max(∆Tb) [K]
CH17
NOAA 15 NOAA 16 NOAA 17
NOAA 15 post Sep. 04 RFI
2000 2001 2002 2003 2004 2005 2006 Time [year]
-2 -1 0 1 2
max(∆Tb) [K]
CH18
NOAA 15 NOAA 16 NOAA 17
NOAA 15 post Sep. 04 RFI
2000 2001 2002 2003 2004 2005 2006 Time [year]
-10 -8 -6 -4 -2 0 2
max(∆Tb) [K]
CH19
NOAA 15 NOAA 16 NOAA 17
NOAA 15 post Sep. 04 RFI
2000 2001 2002 2003 2004 2005 2006 Time [year]
-2 -1 0 1 2
max(∆Tb) [K]
CH20
NOAA 15 NOAA 16 NOAA 17
NOAA 15 post Sep. 04 RFI
Figure 4.7: The time evolution of the maximum asymmetry for three-monthly time periods. The plots from left to right correspond to the AMSU-B channels 16, 17, 18, 19, and 20. Different line styles indicate different satellites: NOAA-15 (thick solid), NOAA-16 (dotted), and NOAA-17 (dashed). The thin solid line is for NOAA-15 where RFI correction coefficients as of August 13, 2004 are manually applied to level 1b data. The diamonds indicate the last, as of writing, RFI coefficients update date. Straight lines show linear fits.
Table 4.1: The range of maximum absolute brightness temperature asymmetries.
This table summarizes Figure 4.7. The maximum of the absolute value of∆Tbwas calculated for three-monthly time periods for each channel and satellite. Given here are the smallest and largest of these numbers.
Channel NOAA-15 NOAA-16 NOAA-17
16 0.98 – 1.45 0.38 – 0.89 0.19 – 0.50
17 0.15 – 2.93 0.22 – 1.07 0.07 – 0.91
18 0.71 – 1.90 0.23 – 0.53 0.28 – 0.49
19 0.88 – 9.67 0.62 – 1.41 0.30 – 0.67
20 0.83 – 2.95 1.21 – 2.13 0.80 – 2.34